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Don't waste time with the MyFico CSRs. It's beyond their paygrade. I would put togther a straightforward cover letter outlining the facts and pointing out the the most likely explanation is some flaw in the database processing. That this occurs on all three CRA FICO 9 scores and is on none of your reports is a clear indication of some systematic error in the FICO algorithm processing they provide to the CRAs. Be helpful. State you want to help in any way possible to assist them in tracking down the problem and that you are concerned it may be affecting others as well. Mail it to the CEO at their corporate headquarters. It will be intercepted and likely routed to someone that will look into it. If you get stonewalled you can file a specific complaint with the CFPB. The most important thing is to outline at the start exactly what you are seeing and why you believe it needs to be tracked down. Keep it simple and clear. I'm sure FICO wants to fix problems like this. It's their primary business.
Just wanted to see if you were able to resolve this. I also have a 60 day deliquency on my report on FICO Score 9, and squeaky clean on FICO Score 8. My FICO Score 9 credit score is 60 points lower than my FICO Score 8. And I cannot find any deliquency. I also got a credit report from annualcreditreport.com and it was also clean.